PHILADELPHIA (AP) _ Chronic fatigue syndrome, a mysterious disabling disease sometimes dismissed as psychosomatic ''yuppie flu,'' has been linked to a family of viruses that disrupts the immune system, researchers said Tuesday.

Evidence of a ''retrovirus'' was found in the blood of 23 of 30 patients suffering from the disease, and in many of their healthy relatives and close friends, the scientists said. That suggests the disease is contagious.

''If it turns out to be the agent causing this, then we learn something about treatment and something about prevention,'' said Walter J. Gunn, who heads the investigation into chronic fatigue syndrome for the federal Centers for Disease Control in Atlanta. ''It has the potential to be a very important study.''

The disease, which may affect millions of Americans, usually starts with flu-like symptoms and can develop into overwhelming fatigue. Other symptoms include muscle and joint pain, headaches, fever, sore throat, memory lapses and depression.

Chronic fatigue syndrome is often misdiagnosed because the symptoms are similar to other diseases. The symptoms, however, linger for months or years.

The findings were presented Tuesday at a neuropathology conference in Kyoto, Japan, by Elaine DeFreitas of the Wistar Institute in Philadelphia and Dr. Paul Cheney of Charlotte, N.C., and Dr. David Bell of Lyndonville, N.Y.

DeFreitas said the researchers still don't fully understand the disease or the role of the retrovirus. Nevertheless, other researchers were excited by the findings.

''It's very encouraging, very important,'' said Marya Grambs, director of the Chronic Fatigue Immune Dysfunction Syndrome Foundation in San Francisco, which estimates 3 million to 10 million Americans have the disease.

''It's so critical that this disease is recognized by the government and medical community so it can be aggressively researched, so people who have it can be validated, and not told they're crazy.''

Some physicians, unable to pinpoint a cause, told many patients their problems were all in their heads, Grambs said. The symptoms can lead to depression and suicide, she said.

Because it was widely reported among well-educated women in their 30s and 40s, chronic fatigue syndrome was dubbed ''yuppie flu.''

Using a new laboratory technique, DeFreitas found that nine of 11 adults and 14 of 19 children had blood cells with a viral sequence similar to the human T-cell lymphotropic retrovirus, specifically HTLV-II.

The HTLV family of viruses is linked with leukemia, chronic diseases of the central nervous system and acquired immune deficiency syndrome.

DeFreitas also checked 10 healthy adults and 10 healthy babies as a control, and none showed the presence of such virus-positive blood cells.

The researchers checked outwardly healthy relatives and close friends of the patients. They found that 43 percent of those exposed to the sick adults tested positively and 29 percent of those exposed to the sick children tested positively.

Cheney noted that retroviruses ordinarily aren't transmitted casually among humans. DeFreitas said the data didn't prove that the agent is casually transmitted. She said more study was needed.

''It's very interesting and it definitely needs to be followed up,'' said Ann Schluederberg, chief of the virology branch of the National Insitutes of Health. She said a more direct cause-and-effect must be established.

''There are things about it that make it not at all implausible - just more has to be done,'' she said.

The CDC is conducting a study to estimate how many people have the disease, Gunn said. The center gets 1,000 to 2,000 phone calls a month from people who suspect they have chronic fatigue syndrome, he said.

The research, Gunn said, must be replicated several times before it is accepted by the scientific and medical community.

''This is an extremely tricky area,'' he said. ''It would not be hard to make a mistake.''